This invention relates generally to a method for transmitting data along a drill string, and more particularly to a method for increasing the data capacity for transmitted data through a length ordered drill string.
Deep subterranean wells are typically drilled using drill strings assembled from 10 meter pipe sections connected end-to-end by heavy threaded tool joints. A drill bit is attached to a drill collar at the downhole end of the drill string, the weight of the collar causing the bit to bite into the earth as the drill string is rotated from the surface. Drilling mud or air is pumped from the surface to the drill bit through an axial hole in the drill string. This fluid removes the cuttings from the hole, provides a hydrostatic head which controls the formation of gases, provides a deposit on the wall to seal the formation, and sometimes provides cooling for the bit.
Communication of information to the surface from downhole sensors of parameters such as pressure, temperature, drilling direction, or formation is desirable. Various methods of communicating that have been tried with varying degrees of success include electromagnetic radiation through the ground formation, electrical transmission through an insulated cable, laser communication through a fiber optic cable, pressure pulse propagation through the drilling mud, and wave propagation through the metal drill string. Each of these methods have advantages and disadvantages associated with signal attenuation, ambient noise, high temperatures and compatibility with standard drilling procedures.
The most commercially viable of these methods has been the transmission of information by pressure pulses in the drilling mud. However, attenuation mechanisms in the mud limit the transmission rate to less then five bits per second.
Acoustic telemetry through the drill string has been the goal of the industry for 50 years. The idea of acoustic telemetry is to produce a modulated elastic waves at the bottom of the well and let it propagate up the drill string to the surface and extract the data from the signal at the surface. At first glance, such a system should work, as the steel drill string is an excellent conductor of sound. However, in practice, these systems do not work. The received signal often does not correspond to the transmitted signal, and working systems would be limited to a low transmission rate on the order of less than 10 Hz.
The theory of acoustic telemetry has been studied to provide an explanation for its unexpectedly poor performance. D. Drumheller, "Acoustical properties of drill string," J. Acoust. Soc. Am. 85, 1048-1064 (1989), analyzed a dispersion equation to determine that the group velocity of the distorted signal through a steel drill string has real roots only in spaced passbands.
The assembled drill string has periodically spaced discontinuities in cross-sectional areas attributed to the tool joints, which form roughly 5% of the length of the drill string and have a cross sectional area 5 times greater than the remainder of the drill string. In U.S. Pat. No. 5,128,901, Drumheller disclosed that the signal loss in a drill string is, in addition to attenuation, also a function of distortion caused by the drill string which was found to function as a comb filter having both stopbands with high attenuation and passbands with minimal attenuation. This behavior resulted from periodic reflections along the drill string at the joint collars. This patent disclosed that the frequency of the acoustic signal should be chosen to fall within the pass bands of the filter, and the signal also should be preconditioned to correct for the distortion.
D. Drumheller, "Attenuation Of Sound Waves In Drill Strings", J. Acoust. Soc. Am. 94 (4), October 1993, pgs 2387-2396, discussed the three types of elastic waves that dominate in acoustic transmission in a drill string at the desired frequency range of 1-2 Khz: extensional, torsional, and bending waves. Communication with bending waves is not feasible because they tend to be the slowest of the three waves and are dispersive even in uniform piping. However, both torsional and extensional waves are long compared to the 125 mm diameter of the common drilling pipe, and these waves tend not to be dispersive in uniform diameter pipe.
Extensional and torsional waves are partially reflected at each of the tool joints. The reflection coefficient of extensional waves depends upon the ratio of the cross-sectional areas while the reflection coefficient of the torsional waves depends upon the ratio of the polar moment of inertia of these cross-sectional areas. Consequently, torsional waves undergo stronger reflections at the tool joints. This is one of the primary reasons for preferring extensional waves as a means for communicating information from downhole to the surface.